This invention relates generally to switched-mode power converters and more particularly it provides a lossless output current sensing in isolated converters using inherent resistance of the output inductor and secondary windings of an isolation transformer.
Switching regulators are widely used to supply power to electronic devices, such as in computers, printers, telecommunication equipment, and other devices. Such switching regulators are available in variety of configurations for producing the desired output voltage or current from a source voltage with or without galvanic isolation. The former are also known as an isolated power converters and the later are called a non-isolated power converters. For all the switching regulators it is common that they have at least one controllable power switch, inductor connected with one end to power switch and second end to output capacitor, control circuit for providing regulation of the output voltage or current and driver for at least one power switch. Control circuit further comprises at least output voltage sensing circuit and either power switch current or inductor current sensing circuit, reference voltage and pulse width modulation (PWM) circuit. A signal proportional to either switch or inductor current is usually used for either overload protection or for implementation of current mode control which has variety of implementations (e.g. peak current mode control, hysteretic current mode control, average current mode control and etc.). Output of PWM circuit is fed into driver circuit which generates appropriate drive signals for power switch or switches. While output voltage and switch current sensing are well known in practice, inductor current sensing has different methods and associated circuits with it.
A signal proportional to the inductor""s current (in most cases output inductor current is sensed) may be used to limit component stress during output overloads (overload protection). However, a regulation application needs a higher fidelity current signal than the overload protection application. Typically thermal design of the switching regulator is sized for maximum efficiency, and the worst case variation of the overload trip level (current signal) still maintains the components below their maximum ratings. Unfortunately, the waveshape may not be suitable for regulation, and in any sensing circuit bandwidth must be sufficient in view of the switching frequency.
Perhaps the most common approach to sensing the output inductor current indirectly in isolated topologies is to use sense resistor in series with power switches. Use of sense resistor in single ended topologies, such as for example forward, flayback and others, as well as in full-bridge and push-pull topologies, allows that one end of sense resistor is connected to GND pin of control chip, usually connected to input return, which simplifies current sensing. On other hand, the sensing resistor value must be large enough to keep the sensed signal above the noise floor and yet small enough to avoid excessive power dissipation. In case of half-bridge converter, for example, this approach is not good since only one primary side power switch is connected to input return and sensed signal does not reflect current through second, floating primary side power switch switch. Using sense resistor in return input path is also not good solution since sensed current is not exactly current through power switches but rather an input current of the converter smoothed by input capacitors. Also, sensed switch current differs from the output inductor current due to magnetizing current of isolation transformer which also varies with the input voltage. In applications where constant output current characteristic is required additional circuitry is needed to compensate for difference between sensed current and output inductor current. For some applications, the value of the sensing resistor may be close or exceed the on resistance of the power switches. Another approach is to use current sense transformer but this approach becomes unacceptable in applications where small size and low profile of the power converter is must.
In cases where control circuit is referenced to the output of the converter or output inductor current is needed to be sensed (e.g. applications where fast transient response to step load changes is required) the most common approach is to the sensing resistor in series with the output inductor. The circuit reconstructs the output inductor current as a differential voltage across the sensing resistor. Most IC""s using this approach regulates output voltage with current mode control and use the signal for output voltage feedback.
The sensing resistor value must be large enough to provide good signal to noise ratio and yet small enough to avoid excessive power dissipation. Since the power dissipated in the sensing resistor increases with the square of the inductor current, this approach has the obvious efficiency drawback with high output current and low output voltage. For low voltage, high current applications, the value of the sensing resistor may be close or even higher than the on resistance of the power switch and inductor which are minimized for maximum efficiency. Thus, sensed signal is relatively small and requires use of more expensive either comparators or amplifiers.
Power inductors are known to have parasitic (or inherent) winding resistance, and therefore can be represented by an equivalent circuit of a series combination of an ideal inductor and a resistor. When direct (DC) current flows through the inductor (or a current having a DC component), a DC voltage drop is imposed across the inductor, which voltage is a product of the magnitude of the DC (component of the) current and the parasitic resistance of the inductor. Since such an inductor may already be present in the circuit, there is no an additional loss of efficiency in using the inductor for this purpose.
U.S. Pat. No. 3,733,536, issued to Gillow and Marple, discloses a current sensor which uses a sense winding with an equivalent number of turns as the filter""s inductor has and is magnetically coupled to it in a voltage canceling relationship. The current sensor provides a sense output signal which is proportional to the filter""s output current and derived substantially from the voltage drop across the effective DC resistance of the inductor. The main drawbacks of this approach are that an additional winding of the output inductor is needed with same numbers of turns as the main winding carrying current which, adds complexity in inductor design (particularly when planar magnetics is used), sensed signal is DC without inductor current ripple information thus, it cannot be used neither for peak current mode control nor suppressing variations in the input voltage (feed-forward) and amplitude of the sensed signal is proportional only to parasitic resistance of the inductor.
Parasitic resistance of the output inductor is used for current sensing without additional winding sensing but rather sensing DC voltage across the output inductor as described in U.S. Pat. No. 5,465,201, issued to Cohen, U.S. Pat. No. 5,877,611, issued to Brkovic, U.S. Pat. No. 5,982,160, issued to Walters et al. and U.S. Pat. No. 6,127,814, issued to Goder, which patents are hereby incorporated herein by reference. Again, sensed signal is limited to product of inductor""s winding resistance and inductor current and can be increased only by means of active amplification, which adds complexity, inaccuracy and mostly additional cost. In order to maximize efficiency of the converter, inductor""s parasitic resistance (particular at high current applications) has to be minimized thus, the sensed signal is relatively small and requires use of more expensive either comparators or amplifiers. Very often error due to offset in comparator and/or amplifier is larger than variation in the winding resistance of the inductor (windings printed on the PCB).
Therefore, it is an object of the present invention to provide an efficient switching regulator and associated methods which provide for accurate sensing of the output current with increased amplitude of the sensed signal in a switched mode power converter for regulation and protection against overload or short circuit conditions.
This and other objects, features and advantages in accordance with the present invention are provided by The switching regulator comprising an input voltage source, a switching circuit and an transformer for coupling the input voltage source and an output stage, the switching circuit comprising at least two controllable power switches and a first and a second rectifier switch, the transformer comprising at least one primary winding connected to the input voltage source via at least two controllable power switches and a first and a second secondary windings, the first and the second secondary winding having inherent resistances, the first and the second secondary windings connected together in series at a first node, the first and the second rectifier switches connected in series at a second node, the first secondary windings connected to the first rectifier switch at a third node, the second secondary winding connected to the second rectifier switch at fourth node, the output stage having a output stage input and a output stage output, the output stage input connected to the first node and the second node, the output stage output connected to a load circuit, the output stage input and the output stage output having a common node, the output stage comprising an inductor for providing output current to the load circuit, the inductor having a first terminal for coupling to the output stage input and a second terminal for coupling to the output stage output, the inductor having an inherent resistance, the output current thereby providing a voltage between the third node and the second terminal of the inductor and a voltage between the fourth node and the second terminal of the inductor including a DC voltage component proportional to the sum of inherent resistance of the first secondary winding and the second secondary winding and inductor and to the output current; a current sensor having a current sensor input and a current sensor output, the current sensor input connected to the third node and to the fourth node and the second terminal of the inductor, the current sensor providing DC voltage component at the current sensor output; the current sensor output connected to a control circuit, the control circuit adjusting an operating parameter of the switching circuit. The current sensor preferably comprises a first and a second resistor and a capacitor, the first resistor having a first and a second terminal, the second resistor having a first and a second terminal, the capacitor having a first and a second terminal, the first terminal of the first resistor coupled to the node three and the first terminal of the second resistor coupled to the node four, the second terminal of the first resistor coupled to the first terminal of the second resistor and the first terminal of the capacitor at a node five, the second terminal of the capacitor coupled to the second terminal of the inductor.
An inductance and a direct current (DC) resistance of the inductor together with a leakage inductance and a DC resistance of the first and the second secondary windings define a first time constant. In one embodiment, the first resistor, second resistor and capacitor of the current sensor define a second time constant within a predetermined range of the first time constant. In another embodiment, the first resistor and the second resistor and capacitor of the current sensor define a second time constant substantially equal to the first time constant.
In one embodiment of the invention the switching regulator may also include a voltage regulation circuit cooperating with the peak current control loop circuit for setting a peak current level. The voltage regulation loop circuit may include a voltage divider connected to the output of the switching regulator for generating a signal related to the output voltage.
The switching regulator may also include a voltage regulation circuit cooperating with the average current control loop circuit for setting an average current level.
According to another aspect of the invention, the current sensor preferably defines a sensing bandwidth substantially above the predetermined switching frequency of the switching regulator.
The switching regulator may also include an overload protection circuit connected to the current sensor. In one embodiment inductor peak current is sensed in order to provide cycle by cycle current limit. In another embodiment average current is sensed in order to provide more precise current limit independent of the operating conditions of the converter.
Another embodiment of the invention relates to a method of providing output current sensing in The switching regulator circuit comprising an input voltage source, a switching circuit and an transformer for coupling the input voltage source and an output stage, the switching circuit comprising at least two controllable power switches and a first and a second rectifier switch, the transformer comprising at least one primary winding connected to the input voltage source via at least two controllable power switches and a first and a second secondary windings, the first and the second secondary winding having inherent resistances, the first and the second secondary windings connected together in series at a first node, the first and the second rectifier switches connected in series at a second node, the first secondary windings connected to the first rectifier switch at a third node, the second secondary winding connected to the second rectifier switch at fourth node, the output stage having a output stage input and a output stage output, the output stage input connected to the first node and the second node, the output stage output connected to a load circuit, the output stage input and the output stage output having a common node, the output stage comprising an inductor for providing output current to the load circuit, the inductor having a first terminal for coupling to the output stage input and a second terminal for coupling to the output stage output, the inductor having an inherent resistance, the output current thereby providing a voltage between the third node and the second terminal of the inductor and a voltage between the fourth node and the second terminal of the inductor including a DC voltage component proportional to the sum of inherent resistance of the first secondary winding and the second secondary winding and inductor and to the output current; the method comprising the steps of providing a current sensor having a current sensor input and a current sensor output, the current sensor input connected to the third node and to the fourth node and the second terminal of the inductor, the current sensor providing DC voltage component at the current sensor output; the current sensor output connected to a control circuit, the control circuit adjusting an operating parameter of the switching circuit. The current sensor preferably comprises a first and a second resistor and a capacitor, the first resistor having a first and a second terminal, the second resistor having a first and a second terminal, the capacitor having a first and a second terminal, the first terminal of the first resistor coupled to the node three and the first terminal of the second resistor coupled to the node four, the second terminal of the first resistor coupled to the first terminal of the second resistor and the first terminal of the capacitor at a node five, the second terminal of the capacitor coupled to the second terminal of the inductor. The current sensor circuit may include offset means to offset the DC voltage component at the current sensor output by a predetermined amount. Also, the current sensor may include means to compensate temperature dependence of the sensed signal due to temperature dependence of the secondary windings and inductor DC resistances.
An inductance and a DC resistance of the inductor and secondary windings define a first time constant. In one embodiment, the method also includes the step of selecting the resistors and capacitor of the current sensor to define a second time constant within predetermined range of the first time constant, or the first and the second time constants may be substantially equal.